The global objective of this proposal is to understand the molecular process by which normal epithelial cells polarize and assemble into a three dimensional (3D) tissue and how this process goes awry in cancers. Using the mammalian epidermis as a model, I aim to (1) determine which polarity gene homologues are expressed in mouse epidermis. (2) Develop a system with which to temporally induce the expression of siRNAs specifically in skin keratinocytes. (3) Target key polarity molecules for knock down by siRNA in primary keratinocytes in vitro and determine the consequences to the organization of cellular architecture in epithelial sheets with particular focus on the structure, composition and dynamics of adherens junctions, desmosomes, actin and microtubule-based cytoskeletons and apical and basal membrane domains. I will also determine the effects of loss of polarity on the balance between keratinocyte proliferation and differentiation. (4) Graft keratinocytes expressing inducible siRNA transgenes on the back skin of a nude mouse and determine how impaired polarity affects proliferation, morphogenesis and migration in the epidermis in vivo.